[CyberPyrot] and [l0rdnic0] released their spitfire mod on acidmods for XBox 360 controllers. It involves some fine wiring, but a relatively low parts count. The heart of the mod is a PIC16F84A, a crystal for the clock and a few passive parts. It took me a little while to find it, but the code for the pic hiding under the parts list. For a lengthy demo of the mod in action, you can check out this video.
Related: Xbox 360 Hacks, Xbox Hacks